Inter-annual zonal shift of the formation region of the lighter variety of the North Pacific Central Mode Water

Inter-annual variability in the formation and subduction of the lighter variety of the Central Mode Water (L-CMW) in the North Pacific is investigated using Argo profile data from 2003 to 2013. The deep mixed layer (ML) in winter (February-April) with the L-CMW property (potential temperature of 10-16 A degrees C and salinity of 34.2-34.6) shows a zonal shift inter-annually. In winters of 2003-2005 and 2010, deep MLs are more frequently generated in the eastern part of the formation region (30A degrees-40A degrees N, 180A degrees-160A degrees W), which is mainly caused by large oceanic heat loss associated with strengthening of the westerlies. Relatively large parts of the L-CMW that formed in this eastern region survive to subduct into the subsurface layer and, in the following summer, spread widely over the North Pacific subtropical gyre. On the other hand, during the winters of 2006-2009 and 2011-2013, deep MLs were more frequently formed in the western part of the formation region (36A degrees-40A degrees N, 140A degrees-155A degrees E). These were induced by weakening of surface ocean stratification attributable to anti-cyclonic meso-scale eddies pinched off northward from the Kuroshio Extension. In contrast to the L-CMW formed in the eastern part of the formation region, the L-CMW formed in the western part is hardly detected in the subsurface layer during the following summer.